createTable and durability level option improvements

- Always create per-table keyspaces using NetworkTopologyStrategy, so that we
  can easily switch to cross-DC replication later.
- Validate schema options
- Support an abstract durability level. For now, only 'low' and 'standard' are
  supported.
- Make table creation conditional to handle a race condition between multiple
  workers on setup
Merge pull request #17 from wikimedia/createTable

createTable and durability level option improvements